Thermal shock-resistant silicon nitride sintered material

ABSTRACT

There is provided a thermal shock-resistant silicon nitride sintered material consisting substantially of silicon nitride and rare earth element compounds, which material contains at least 10 pore groups per mm 2 , each pore group consisting of pores of 10 μm or less and which material has a thermal shock resistance ΔTc (°C.) of 1,000 ° C. or more. The thermal shock-resistant silicon nitride sintered material can be produced by mixing and shaping starting materials consisting of a silicon nitride powders of rare earth element oxides and carbide power, and then firing the shaped material in a nitrogen atmosphere to decompose the carbide powders.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a silicon nitride sintered materialhaving a high-temperature strength, excellent thermal shock resistanceand a low Young's modulus, as well as to a process for producing thesintered material.

With respect to silicon nitride sintered materials containing oxides ofIIIa group elements including rare earth elements, for example, JapanesePatent Publication No. 7486/1973 discloses a process for producing asintered material, which comprises mixing and shaping 85 mole % or moreof silicon nitride (Si₃ N₄) and 15 mole % or less of at least one oxideselected from oxides of IIIa group elements and then sintering theshaped material in a non-oxidizing atmosphere. Japanese PatentPublication No. 21091/1974 discloses a silicon nitride sintered materialconsisting of 50% by weight of Si₃ N₄, 50% by weight or less of at leastone oxide selected from Y₂ O₃ and oxides of La type elements and0.01-20% by weight of Al₂ O₃.

However, there have been problems that addition of only rare earthelements to silicon nitride fails to provide a sintered material havinga high-temperature strength and addition of Al₂ O₃ provides a sinteredmaterial which has a higher density but whose grain boundary phase has alower melting point and gives a very low high-temperature strength.

In order to solve the problem of inadequate high-temperature strength,Japanese Patent Application Kokai (Laid-Open) No. 100067/1988 disclosesa technique for achieving a high-temperature strength by adding rareearth elements of given composition and given proportion to a Si₃ N₄powder and sintering the mixture to allow the resulting sinteredmaterial to have a specific crystalline phase.

The silicon nitride sintered material disclosed in Japanese PatentApplication Kokai (Laid-Open) No. 100067/1988 can achieve ahigh-temperature strength to some extent but has problems that theYoung's modulus is as large as 300 GPa and the thermal shock resistanceΔTc (°C.) is as small as 1,000° C. This is because the silicon nitridesintered material is homogeneous microscopically, and has a Young'smodulus and thermal expansion coefficient characteristic of siliconnitride and, as a result, the thermal shock resistance ΔTc (°C.) of thesilicon nitride sintered material is substantially determined dependingupon its strength. The thermal shock resistance ΔTc (°C.) of a ceramiccan be evaluated by the following formula.

    ΔTc∝σ/αE

[σ is a flexural strength (Pa), α is a thermal expansion coefficient(1/°C.), and E is a Young's modulus (Pa).]

The object of the present invention is to solve the above-mentionedproblems and provide a silicon nitride sintered material having ahigh-temperature strength about equal to the room temperature strengthand excellent thermal shock resistance, as well as a process forproducing the sintered material.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a thermalshock-resistant silicon nitride sintered material consistingsubstantially of silicon nitride and rare earth element compounds, whichmaterial contains at least 10 pore groups per mm², each pore groupconsisting of pores of 10 μm or less in diameter and having a diameterof 30-100 μm and which material has a thermal shock resistance ΔTc (°C.)of 1,000° C. or more.

The present invention further provides a process for producing a thermalshock-resistant silicon nitride sintered material, which processcomprises mixing and shaping starting materials consisting of a siliconnitride powder, powders of rare earth element oxides and carbidepowders, and then firing the shaped material in a nitrogen atmosphere to(a) react the carbide powders with the silicon oxide present in thesilicon nitride to convert the carbide to nitrides or silicides andsimultaneously give rise to decomposition gases and (b) thereby form, inthe resulting sintered material, pores of 10 μm or less in diameter andpore groups each consisting of said pores and having a diameter of30-100 μm.

In the present invention, the sintered material obtained can besubjected to a heat treatment of 1,000°-1,500° C. in anoxygen-containing atmosphere to form, on the surface, a surface layer of5-100 μm in thickness consisting of silicates of rare earth elements andsilicon oxide, whereby a silicon nitride sintered material of higherstrength and higher thermal shock resistance can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing the microstructure of thesilicon nitride sintered material according to the present invention.

FIG. 2 is a micrograph of the silicon nitride sintered material ofExample 1 according to the present invention after the polished surfacehas been oxidized.

FIG. 3 is a micrograph of the silicon nitride sintered material ofComparative Example 1 after the polished surface has been oxidized.

FIG. 4 is a micrograph of the silicon nitride sintered material ofComparative Example 3 after the polished surface has been oxidized.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, a silicon nitride powder containing powders ofgiven rare earth element oxides is mixed with carbide powders and theresulting mixture is fired in a nitrogen atmosphere, whereby a siliconnitride sintered material which contains at least 10 pore groups permm², is obtained, each pore group consisting of micropores of 10 μm orless in diameter and having a diameter of 30-100 μm and whichaccordingly has a unique microstructure. This silicon nitride sinteredmaterial has a high-temperature strength about equal to the roomtemperature strength and excellent thermal shock resistance.

The silicon nitride as a starting material contains oxygen as animpurity, and this oxygen is present in the form of SiO₂. By utilizingthe reaction of this SiO₂ with carbides in a nitrogen atmosphere, forexample, the following reaction, decomposition gases can be generated inthe resulting sintered material.

    2SiC+SiO.sub.2 +2N.sub.2 →Si.sub.3 N.sub.4 +2CO↑(1)

    WC+2SiO.sub.2 →WSi.sub.2 +CO↑+3/2O.sub.2 ↑(2)

In the present invention, by adding carbides to silicon nitride anddecomposing the carbide as above at a temperature close to the sinteringtemperature, there can be obtained a silicon nitride sintered materialhaving a unique structure as shown in FIG. 1, containing at least 10pore groups per mm² of sintered material, each consisting of microporesof 10 μm or less in diameter and having a diameter of 30-100 μm. In FIG.1, the numeral 1 refers to a sintered material; the numeral 2 refers tomicropores; and the numeral 3 refers to a pore group.

Such a silicon nitride sintered material containing groups of microporescauses no deterioration in strength, unlike sintered materialscontaining ordinary pores. The presence of micropore groups is effectivefor reduction in Young's modulus, making it possible to obtain a siliconnitride sintered material of high strength and low Young's modulus. Asis known, the thermal shock resistance of a ceramic can be evaluated byits ΔTc (°C.), and a larger ΔTc value gives higher thermal shockresistance. When, for example, a material heated to 1,000° C. is placedinto cool water of 0° C. and thereby the material begins to crack, etc.and exhibits a resultant reduction in strength, the ΔTc (°C.) of thematerial is defined to be 1,000° C. In general, the ΔTc of a ceramic isgiven by the following formula

    ΔTc∝σ/αE

[σ is a flexural strength (Pa), α is a thermal expansion coefficient(1/°C.), and E is a Young's modulus (Pa).], and is dependent upon thestrength, thermal expansion coefficient and Young's modulus of theceramic. Therefore, when the strength is constant, as the Young'smodulus is lower, the ΔTc is larger and the thermal shock resistance ishigher, providing a more useful material.

The silicon nitride sintered material of the present invention is justsuch a material, and has a reduced Young's modulus without sacrificingthe strength and accordingly has significantly improved thermal shockresistance.

In the present invention, the oxygen amount in the silicon nitride usedas a starting material is desirably 1-3% by weight. This oxygen amountcan be controlled by oxidizing the silicon nitride, or by adding a SiO₂powder to the silicon nitride.

The total amount of the rare earth element oxides used also as astarting material is preferably 6-21% by weight. When the total amountis less than 6% by weight, it is impossible to obtain a liquid phasenecessary for achieving a high density. When the total amount is morethan 21% by weight, it tends to be difficult to achieve a high density.As the rare earth element oxides other than Y₂ O₃ and Yb₂ O₃, there canalso be used Lu₂ O₃, Tm₂ O₃, Er₂ O₃, etc. The total amount of the rareearth elements in sintered material is the same as that in startingmaterials.

The amount of the carbides used also as a starting material ispreferably 0.5-10% by weight. An amount of less than 0.5% by weight isnot effective to sufficiently form pore groups. An amount of more than10% by weight hinders the achievement of a high density in some cases.The amount of the carbide is more preferably 1-5% by weight. The Ccontent in sintered material is 1/2 or less of the C content in startingmaterials. Incidentally, the type of carbide is not restricted by thecrystalline phase; and, in the case of SiC, there can be used any of αtype, β type and amorphous type.

In the process for producing a silicon nitride sintered materialaccording to the present invention, at first there is prepared a mixtureof a silicon nitride powder, powders of rare earth element oxides andcarbide powders. The mixture is then made into a desired shape to obtaina shaped material. Thereafter, the sintered material is fired at1,700°-2,100° C., preferably 1,900°-2,000° C. for 3-6 hours in anitrogen atmosphere of normal pressure to applied pressure correspondingto the firing temperature to (a) react the carbide powders with thesilicon oxide present in the silicon nitride to convert the carbide tonitride or silicide and thereby (b) generate the decomposition gases toform pores of 10 μm or less in diameter and simultaneously groups ofsaid pores, each having a diameter of 30-100 μm, whereby a siliconnitride sintered material of the present invention can be obtained.

It is possible that the thus obtained silicon nitride sintered materialitself or after having been subjected to grinding, etc. so as to have adesired shape, be subjected to a heat treatment at 1,000°-1,500° C. inan oxygen-containing atmosphere to form, on the surface, a surface layerof 5-100 μm in thickness consisting of silicates of rare earth elementsand silicon oxide and thereby obtain a silicon nitride sintered materialfurther improved in thermal shock resistance.

The present invention is hereinafter described in more detail by way ofExamples. However, the present invention is in no way restricted tothese Examples.

EXAMPLES AND COMPARATIVE EXAMPLES

There were mixed (a) a silicon nitride powder having a purity of 97% byweight, an oxygen content of 2.2% by weight, an average particlediameter of 0.6 μm and a BET specific surface area of 17 m² /g, (b)powders of oxides shown in Table 1, having a purity of 99.9% by weightand an average particle diameter of 0.3-2.5 μm and (c) powders ofcarbides shown in Table 1, such as SiC, WC, Mo₂ C, TiC, NbC, TiC and thelike, having a purity of 99% by weight and an average particle diameterof 0.4-3 μm, in proportions shown in Table 1. 200 g of the resultingmixture, 1,800 g of pebbles made of a silicon nitride ceramic and 300 mlof water were placed in a 1.2-l nylon container, and the container wasplaced in a vibration mill and subjected to vibration of 1,200 times/minfor 3 hours to grind the mixture. Then, water was vaporized and theground mixture was granulated to a particle diameter of 150 μm to obtaina powder for shaping. Thereafter, the powder was subjected to coldisostatic pressing at a pressure of 7 ton/cm² to prepare a shapedmaterial of 50×40×6 mm, and the shaped material was fired under thefiring conditions shown in Table 1 to obtain silicon nitride sinteredmaterials of Examples 1-15 of the present invention. The sinteredmaterials of Examples 10-12 were further subjected to a heat treatment.

Separately, there were mixed the same silicon nitride powder as above,powders of oxides shown in Table 2 and a powder of a carbide shown inTable 2, in proportions shown in Table 2; the resulting mixture wassubjected to the same grinding, granulation and shaping as above; theresulting shaped material was fired under the firing conditions shown inTable 2 to obtain sintered materials of Comparative Examples 1-6.

Each of the above obtained sintered materials was measured for bulkdensity, crystalline phase of grain boundary, 4-point flexural strengthsat room temperature, 1,000° C. and 1,400° C., Young's modulus at roomtemperature and thermal shock resistance ΔTc (° C.). The results areshown in Tables 1 and 2. In Tables 1 and 2, bulk density of sinteredmaterial was measured by the Archimedes' method. This density wasexpressed in the tables as a value relative to theoretical density,wherein the theoretical density was calculated from composition of mixedpowder and its density. In calculation of density of mixed powder, therewere used Si₃ N₄ : 3.2 g/cm³, Y₂ O₃ : 5.0 g/cm³, Yb₂ O₃ : 9.2 g/cm³, Tm₂O₃ : 8.8 g/cm³, Lu₂ O₃ : 9.4 g/cm³, Er₂ O₃ : 8.6 g/cm³, SiC: 3.2 g/cm³,WC: 15.6 g/cm³, Mo₂ C: 8.9 g/cm³, TiC: 4.9 g/cm³, TaC: 14.7 g/cm³, andNbC: 7.8 g/cm³. 4-Point flexural strength was measured in accordancewith JIS R 1601 "Test Method for Flexural Strength (Modulus of Rupture)of High Performance Ceramics". Crystalline phase of grain boundary wasdetermined from the results of X-ray diffraction by CuK α-rays. InTables 1 and 2, J is a cuspidine type crystal and gives the samediffraction pattern as Si₃ N₄ ·4Y₂ O₃ ·SiO₂ represented by JCPDS card32-1451, wherein the crystallographic position of Y can be replaced byother rare earth elements. H is an apatite type crystal and gives thesame diffraction pattern as Si₃ N₄ ·10Y₂ O₃ ·9SiO₂ represented by JCPDScard 30-1462, wherein the crystallographic position of Y can be replacedby other rare earth elements. K is a wollastonite type crystal and givesthe same diffraction pattern as 2Y₂ O₃ ·SiO₂ ·Si₃ N₄ represented byJCPDS card 31-1462, wherein the crystallographic position of Y can bereplaced by other rare earth elements. L is a crystal represented by Re₂SiO₅ (Re: rare earth element) and gives the same diffraction pattern asany of JCPDS cards 21-1456, 21-1458, 21-1461, 22-992 and 36-1476. S is acrystal represented by Re₂ SiO₇ (Re: rare earth element) and gives thesame diffraction pattern as any of JCPDS cards 20-1416, 21-1457,21-1459, 21-1460, 22-994 and 22-1103. C is a SiO₂ (cristobalite) crystaland gives diffraction patterns represented by JCPDS cards 11-695 and27-605. The amount of each crystalline phase was determined from themaximum height of the diffraction pattern. In Tables 1 and 2, ">"(inequality sign) refers to "larger" or "smaller" in amount ofcrystalline phase, and "," refers to "about equal" in amount ofcrystalline phase.

Young's modulus (room temperature) was measured by a pulse echo overlapmethod using a columnar sample of 10 mmφ×20 mm (length). Thermal shockresistance ΔTc (° C.) was measured by an in-water quenching methodwherein a test piece heated to a given temperature was placed into waterto quench it and the reduction in room temperature strength of the testpiece was examined.

Average pore diameter and pore group diameter were measured bysubjecting a sintered material to polishing to allow the material tohave a mirror surface and subjecting the mirror surface to imageanalysis using an optical microscope. Pore group density was determinedby subjecting a sintered material to the same polishing, subjecting theresulting material to an oxidation treatment in air, and then measuringthe number of pore groups per unit area. The oxidation treatment enableseasy measurement of number of pore groups. The size of each pore grouplooks larger than the actual diameter of pore group.

FIG. 2 is an photomicrograph of the silicon nitride sintered material ofExample 1 of the present invention after the polished surface has beenoxidized. In the photomicro-graph, white spots are groups of pores.

Meanwhile, FIG. 3 is an photomicrograph of the silicon nitride sinteredmaterial of Comparative Example 1 after the polished surface has beenoxidized, wherein no white spot having a diameter of 30 μm or more isseen.

FIG. 4 is an photomicrograph of the silicon nitride sintered material ofComparative Example 3 after the polished surface has been oxidized,wherein pore groups of abnormally large size are seen.

                                      TABLE 1                                     __________________________________________________________________________                    Example No.                                                                   1   2   3    4   5     6    7    8                            __________________________________________________________________________    Powders mixed                                                                 Proportions (wt %)                                                            Y.sub.2 O.sub.3 3.4 3.2 --   2.0 2.0   2.0  9.9  4.2                          Yb.sub.2 O.sub.3                                                                              13.9                                                                              13.3                                                                              14.9 8.9 6.9   4.0  0    16.8                         Other(s)                                                                      Carbide(s)      Sic 1                                                                             SiC 5                                                                             SiC 0.5                                                                            WC 1                                                                              Mo.sub.2 C 1                                                                        SiC 0.5                                                                            TiC 1                                                                              NbC 1                                                               Mo.sub.2 C 0.5                         Chemical analysis                                                             C content (wt %)                                                                              0.35                                                                              1.55                                                                              0.20 0.11                                                                              0.10  0.23 0.25 0.13                         O content (wt %)                                                                              4.53                                                                              4.35                                                                              4.55 3.76                                                                              3.41  3.02 4.19 4.93                         Sintered material                                                             Chemical analysis                                                             C content (wt %)                                                                              0.05                                                                              0.75                                                                              0.10 0.04                                                                              0.04  0.05 0.03 0.02                         O content (wt %)                                                                              3.98                                                                              3.24                                                                              3.43 3.26                                                                              2.95  2.52 3.39 4.43                         Structure of sintered material                                                Average pore diameter (μm)                                                                 5   10  2    2   2     3    3    3                            Pore group diameter (μm)                                                                   70  100 30   20  20    30   35   30                           Pore group density (per mm.sup.2)                                                             30  50  15   10  11    16   20   15                           Surface layer thickness (μm)                                                               --  --  --   --  --    --   --   --                           Crystalline phase of surface layer                                                            --  --  --   --  --    --   --   --                           Mechanical properties                                                         Flexural strength R.T. (MPa)                                                                  820 800 750  850 780   690  790  710                                   1000° C. (MPa)                                                                820 800 750  850 780   690  790  710                                   1400° C. (MPa)                                                                820 800 740  840 750   670  760  700                          Young's Modulus R.T. (GPa)                                                                    260 250 270  270 270   270  270  270                          Thermal shock resistance (°C.)                                                         1300                                                                              1300                                                                              1100 1300                                                                              1150  1000 1200 1050                         Relative density of                                                                           99  99  97   97  99    99   99   99                           sintered material (%)                                                                         J   J   J    H   H > J L > S                                                                              J    J                            Crystalline phase of                                                          grain be undary                                                               Firing conditions                                                             Firing temperature (°C.)                                                               1900                                                                              1900                                                                              1700 2100                                                                              1900  1900 1950 1900                         Firing time (hr)                                                                              4   4   6    2   4     4    3    4                            Nitrogen pressure (atm)                                                                       10  10  1    100 10    10   50   10                           Heat treatment conditions                                                     Heat treatment temperature (°C.)                                                       --  --  --   --  --    --   --   --                           Heat treatment time (hr)                                                                      --  --  --   --  --    --   --   --                           __________________________________________________________________________                    Example No.                                                                   9   10   11   12   13    14    15                             __________________________________________________________________________    Powders mixed                                                                 Proportions (wt %)                                                            Y.sub.2 O.sub.3 3.4 3.4  3.4  3.4  1.9   --    1.9                            Yb.sub.2 O.sub.3                                                                              13.5                                                                              13.9 13.9 13.9 --    6.7   --                             Other(s)                           Tm.sub.2 O.sub.3 8.6                                                                Lu.sub.2 O.sub.3                                                                    Er.sub.2 O.sub.3 12.5          Carbide(s)      SiC 10                                                                            Sic 1                                                                              Sic 1                                                                              Sic 1                                                                              Mo.sub.2 C 4                                                                        TaC 4 Mo.sub.2 C 4                   Chemical analysis                                                             C content (wt %)                                                                              3.05                                                                              0.35 0.35 0.35 0.26  0.30  0.26                           O content (wt %)                                                                              4.28                                                                              4.53 4.53 4.53 3.48  3.68  4.01                           Sintered material                                                             Chemical analysis                                                             C content (wt %)                                                                              1.50                                                                              0.05 0.05 0.05 0.03  0.09  0.04                           O content (wt %)                                                                              2.98                                                                              3.98 3.98 3.98 2.88  3.14  3.48                           Structure of sintered material                                                Average pore diameter (μm)                                                                 10  5    5    5    8     5     4                              Pore group diameter (μm)                                                                   100 70   70   70   80    70    50                             Pore group density (per mm.sup.2)                                                             100 30   30   30   50    40    50                             Surface layer thickness (μm)                                                               --  30   100  20   --    --    --                             Crystalline phase of surface layer                                                            --  S > L, C                                                                           S > L, C                                                                           S > L, C                                                                           --    --    --                             Mechanical properties                                                         Flexural strength R.T. (MPa)                                                                  750 850  850  750  770   780   800                                     1000° C. (MPa)                                                                750 850  850  750  770   780   800                                     1400° C. (MPa)                                                                740 850  850  740  750   760   780                            Young's Modulus R.T. (GPa)                                                                    250 260  260  270  250   250   250                            Thermal shock resistance (°C.)                                                         1200                                                                              1350 1350 1100 1200  1200  1300                           Relative density of                                                                           95  99   99   97   98    98    98                             sintered material (%)                                                         Crystalline phase of                                                                          J   J, H J, H J, H H > L J     J                              grain be undary                                                               Firing conditions                                                             Firing temperature (°C.)                                                               1900                                                                              1900 1900 1900 2000  1900  1900                           Firing time (hr)                                                                              6   4    4    4    3     4     4                              Nitrogen pressure (atm)                                                                       10  10   10   10   100   10    10                             Heat treatment conditions                                                                     --                                                            Heat treatment temperature (°C.)                                                       --  1300 1500 1000 --    --    --                             Heat treatment time (hr)                                                                      --  1    1    5    --    --    --                             __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                    Comparative Example No.                                                       1    2    3    4    5    6                                    __________________________________________________________________________    Powders mixed                                                                 Proportions (wt %)                                                            Y.sub.2 O.sub.3 3.4  2.2  2.0  5    2.9  5.0                                  Yb.sub.2 O.sub.3                                                                              14.0 8.8  8.9  --   11.9 19.0                                 Other(s)        --   --   --   MgO 3                                                                              --   --                                                                  ZrO.sub.2 0.3                                  Carbide(s)      SiC 0.1                                                                            --   SiC 1                                                                              SiC 1                                                                              SiC 15                                                                             SiC 1                                Chemical analysis                                                             C content (wt %)                                                                              0.08 0.05 0.35 0.35 4.54 0.35                                 O content (wt %)     3.80 4.53 4.60 3.90 5.40                                 Sintered material                                                             Chemical analysis                                                             C content (wt %)                                                                              0.05 0.05 0.02 0.32 4.04 0.06                                 O content (wt %)     3.40 3.70 4.48 3.07 4.50                                 Structure of sintered material                                                Average pore diameter (μm)                                                                 2    2    10   2    15   10                                   Pore group diameter (μm)                                                                   20   No pore                                                                            250  No pore                                                                            No pore                                                                            No pore                              Pore group density (per mm.sup.2)                                                             5    group                                                                              10   group                                                                              group                                                                              group                                Surface layer thickness (μm)                                                               --   --   --   --   --   --                                   Crystalline phase of surface layer                                                            --   --   --   --   --   --                                   Mechanical properties                                                         Flexural strength R.T. (MPa)                                                                  770  700  500  1000 500  600                                           1000° C. (MPa)                                                                770  650  500  1000 500  --                                            1400° C. (MPa)                                                                750  600  500  300  --   --                                   Young's Modulus R.T. (GPa)                                                                    300  300  250  270  250  --                                   Thermal shock resistance (°C.)                                                         800  800  500  900  500  --                                   Relative density of                                                                           97   96   99   98   80   85                                   sintered material (%)                                                         Crystalline phase of                                                                          J > H                                                                              H > S                                                                              J    J    J > H                                     grain be undary                                                               Firing conditions                                                             Firing temperature (°C.)                                                               1900 1900 1900 1700 1850 1900                                 Firing time (hr)                                                                              2    2    10   1    5    2                                    Nitrogen pressure (atm)                                                                       10   10   10   1    5    10                                   Heat treatment conditions                                                     Heat treatment temperature (°C.)                                                       --   --   --   --   --   --                                   Heat treatment time (hr)                                                                      --   --   --   --   --   --                                   __________________________________________________________________________

As is seen from Tables 1 and 2, the sintered materials of Examples 1-15of the present invention using rare earth element oxides as a sinteringaid and carbides, have a high relative density of 95% or more, a lowYoung's modulus of 270 GPa or less and a high thermal shock resistanceΔTc (° C.) of 1,000° C. or more. Meanwhile, the sintered materials ofComparative Examples 1 and 2 using a carbide in an amount of less than0.5% by weight, have pore groups of less than 30 μm in diameter andaccordingly have a high Young's modulus of 300 GPa and a low thermalshock resistance ΔTc (° C.) of less than 1,000° C. although they haveabout the same strength as the sintered materials of the presentinvention.

The sintered material of Comparative Example 5 having an average porediameter of more than 10 μm and the sintered material of ComparativeExample 3 having pore groups of more than 100 μm in diameter, have a lowstrength and accordingly a low thermal shock resistance ΔTc (° C.)although they have a low Young's modulus.

Further, as appreciated from Examples 10-12, when the sintered materialof the present invention is heat-treated at 1,000°-1,500° C. in air, asurface layer consisting of silicates of rare earth elements and siliconoxide is formed on the surface of the sintered material, and such asintered material has a higher strength and higher thermal shockresistance.

What is claimed is:
 1. A thermal shock-resistant silicon nitridesintered material consisting essentially of silicon nitride and rareearth element compounds, which material contains at least 10 pore groupsper mm², each pore group consisting of pores of 10 μm or less indiameter and having a diameter of 30-100 μm and which material has athermal shock resistance ΔTc (° C.) of 1,000° C. or more.
 2. A thermalshock-resistant silicon nitride sintered material according to claim 1,which has a surface layer of 5-100 μm in thickness consisting ofsilicates of rare earth elements and silicon oxide.